Feeding of material to be treated in a treatment vessel may be carried out in different ways and is dependent on a number of factors such as the characteristics of the material to be fed and the possible desired action on the material beside the actual feeding thereof. Plug screw feeders, in which a rotating screw transports the material forward is a commonly used type of feeder. At the same time as the material is fed to the treatment vessel, the screw and associated screw pipe or housing also exercises a volumetric compression function. Such plug screw feeders are very commonly used for feeding wood chips to a digester or an impregnator. Commonly, such plug screw feeders also comprise a plug pipe at the end of the feeder, with an essentially constant inner diameter. The compression taking place in that part of the plug screw feeder is essentially axial.
A known problem with such plug screw feeders is that the theoretical possible compression of the material in the plug screw feeder is often not obtained. A solution to such a problem may be to “force-feed” the plug screw, i.e. making sure that material is fed to the plug screw such that a main part of the available volume in the screw housing is properly filled and thus may be compressed.
A prior art screw feeder apparatus using the principle of force-feeding is disclosed in U.S. Pat. No. 5,320,034. The known apparatus is to be used for increasing fiber dislocation of wood chips, improving a subsequent impregnation of the fiber material. By forcing material into the inlet section of the compression device (plug screw feeder) by means of a force-feeding screw, the packing density, i.e. the degree of filling, will be increased and uniform in the inlet of the plug screw. In other words, it is ensured that the plug screw feeder volume at the inlet section is filled with wood chips.
However, when dealing with other types of materials, different considerations may apply. It has been discovered that the screw feeder apparatus disclosed in U.S. Pat. No. 5,320,034 is not applicable to the feeding of non-wood plant material, such material having a much lower bulk density than wood chips, without modification.
The use of non-wood plants has recently become more and more important. In some cases the material is used as fuel for generating heat. The material may also be used for production of pulp for paper-making purposes or the like, replacing wood as the source material. In many such processes, the non-wood material is treated in a pressurized reactor, imposing special requirements on the feeding arrangement.
There is thus a need for an improved feeding arrangement suitable for the feeding of bulky non-wood plant material and accordingly a system for treatment of such material comprising such a feeding arrangement and a method for feeding non-wood plant material.